1. Field of the Invention
This invention pertains to an incremental speed and/or position sensor which utilizes a probe which is sensitive to the direction and amplitude of a magnetic field, a Hall-effect gaussmeter for example, for the detection of speed and/or position, which is capable of detecting speeds down to a low value corresponding to a null speed, and which can implement a laser trimming operation to enhance operation in the sensor.
2. Discussion of the Background
The magnetic probe delivers an electric signal as a function of the magnetic field crossing it. The state-of-the-art speed sensors are designed in such a manner that the displacement of a moving component will cause a variation of the magnetic field. Subsequently in the description we will use the term "Hall-effect gaussmeter" to designate a sensor which is sensitive to the direction and amplitude of the magnetic field, which provides an electric signal as a function of the direction and amplitude of the magnetic field crossing it, without this term having to be interpreted in any restrictive way.
The first type of Hall-effect sensor includes a moving component with an alternation of north and south magnetic poles. The Hall-effect gaussmeter is connected to an electronic circuit with a logical output, which assumes a first state when the comparison unit located at the output of the Hall-effect gaussmeter provides a signal greater than a positive predetermined threshold, and a second state when the Hall-effect gaussmeter provides a signal lower than a negative predetermined threshold. Such a device is described, for example, in the French patent FR 2,648,914. Such sensors are sensitive to different external disturbances, and the dimensions of the moving component are limited by its cost.
Another type of Hall-effect sensor is comprised of a toothed moving component made of a soft magnetic material, a permanent magnet and a Hall-effect gaussmeter placed between the toothed surface of the moving component and the permanent magnet. Such sensors are, for example, described in the certificate of use American patent U.S. Pat. No. 5,250,925. Such sensors are poorly adapted for the use of a toothed surface with small amount of pitch. In order to reduce the detection pitch it is therefore necessary to move the magnet closer to the toothed surface, which creates mechanical constraints which are difficult to adhere to during fabrication of very great numbers of devices.
Such sensors are particularly poorly adapted for the detection of low speeds, even more so for the detection of position, because of the average increased value of the field with respect to the amplitude of its modulation. This average value and its conversion to an electric signal exhibits great sensitivity to various tolerances which are difficult to control or to compensate for, especially:
the B(H) characteristics of the magnet in the second quadrant; PA1 tolerances of the magnet dimensions; PA1 the coefficients of residual induction temperature Br and of the coercive field Hc of the magnet; PA1 more or less good saturation of the magnet during magnetization; PA1 tolerances for the geometry of the teeth arrangement; PA1 the distance between the magnet and the teeth; PA1 tolerances for the position of the Hall-effect gaussmeter; PA1 offset and different coefficients at temperature of the Hall-effect gaussmeter. PA1 B.sub.total is the component of induction along OZ PA1 the origin O of the axes, the beginning from which the distance z is measured, is located on the side of the magnet opposite the teeth; PA1 B.sub.r designates the residual induction; PA1 B.sub.solid magnet designates the induction of an unperforated virtual magnet, especially a cylindrical one, of which the external dimensions correspond to the external dimensions of the permanent magnet; PA1 B.sub.virtual magnet designates the induction of a virtual magnet whose external dimensions correspond to the internal dimensions of the cavity; PA1 B.sub.plug magnet designates the induction of a virtual magnet in which the section corresponds to the internal section of the cavity, and in which the height corresponds to the difference between the actual magnet and the depth of the cavity. PA1 l designates the difference between the thickness of the magnet and the depth of the cavity (l is null in the case of an air gap which crosses the magnet); PA1 L designates the thickness of the magnet in the case in which the magnet is connected to a large size yoke, the yoke then playing the role of magnetic reflector. If the magnet is not connected to a yoke, the thickness of the magnet is 2L; PA1 z designates the ordinate of the Hall-effect gaussmeter measured from an origin located on the side of the magnet opposite the teeth; PA1 R designates the external radius of the magnet; PA1 r designates the radius of the cavity or of the cylindrical air gap.
In the description which follows "YOZ plane" will designate the plane of symmetry of the teeth of the moving component. OY will designate the direction of displacement of the moving component. O will designate the point on the envelope of the teeth tips located on the right side which pass over the center of the Hall-effect gaussmeter and of the magnet. When the moving component is a toothed wheel the center O' of the toothed wheel is located on the OX axis.
The XOZ plane will designate the plane of symmetry of the tooth, or of the tooth hollow space located opposite the Hall-effect gaussmeter and the permanent magnet. OZ designates the axis which passes by the Hall-effect gaussmeter, the magnet and the toothed surface. OX is generally a radial axis.
In the case of a rotating sensor, XOY is a plane which is tangential to the surface of a toothed wheel which comprises the moving component, and OX is generally an axis which is parallel to the generatrix of the teeth.
In the case of a linear sensor, XOY is the central plane of the toothed surface.
The electronic circuit which processes the signal provided by the Hall-effect gaussmeter, arranged in order to detect the ingredient of the field along OZ, can include a comparison unit which is sensitive to the amplitude of the signal which has a constant sign, with the aforementioned disadvantages regarding fluctuations of its average value. But most frequently the electronic circuit will include a high-pass filter in front of the comparison unit which eliminates the average value of the signal in order to allow only its modulation to pass through. Without this filter, the modulation could be sufficient without the comparison unit being activated.
Within the state of the art there has been a suggestion to remedy the disadvantages which occur at low speed, or the impossibility of functioning as a position detector by different solutions which have not turned out to be totally satisfactory.
A first solution is described in the patent PCT WO 87/06348 pertaining to a sensor which detects not only variation of a magnetic field, but also its average value, which requires the use of a filter which does not function at very low speeds. Even for a speed which suddenly reaches an increased value the filter only gives the average value at the end of several measurement periods. Therefore there might be some detection errors at the start.
A second solution, which is described in the patent BE 0,363,512, consists of utilizing two Hall-effect gaussmeters to generate a differential sensor.
These two solutions require complex electronic circuits, or large area silicon chips, which greatly increase the cost of the sensors, particularly in the case when one is using a differential sensor.
A third solution is described in the American patent U.S. Pat. No. 4,481,469 which describes a sensor which detects the tangential component of the field along the OY axis. This component changes sign during displacement by a value corresponding to one pitch, and its average value is zero. This solution admittedly allows one to partially make up for the disadvantages of the sensors conforming to the two preceding solutions. However, such a sensor is no longer completely satisfactory. The amplitude of the component of the magnetic field detected by the Hall-effect gaussmeter is weak, especially near the magnet. In addition, after the Hall-effect gaussmeter is placed in the XOZ plane and is no longer in a plane parallel to XOY, the distance between the magnet and the teeth must be increased with respect to the other solutions. This increase of the distance also has an unfavorable impact on the amplitude of the detected view, and the sensor is for this reason sensitive to parasitic magnetic fields.
Other solutions pertain to sensors in which the Hall-effect gaussmeter is placed opposite a slit provided in a metallic part. The patent U.S. Pat. No. 5,321,355 pertains to a position sensor which utilizes a Hall-effect gaussmeter. The Hall-effect gaussmeter is arranged in proximity of a metallic strip which has a slit. In the sensor, according to this technological level, the slit provided in the metal strip is intended to convert the increased induction of the permanent magnet to a weak induction which is compatible with a Hall-effect gaussmeter.
The state of the art techniques also include the German patent DE 3,638,622 in which the Hall-effect gaussmeter is in the channel of a ring-shaped magnet. A ring-shaped magnet creates a field that has a null field zone arranged approximately on the central axis of the magnet at a point localized on the external area of the ring-shaped magnet. This point is, moreover, more distant with respect to the frontal plane of the ring-shaped magnet since the diameter of the central channel is large and the magnet is short. For example, for a 10 mm diameter magnet, 4 mm in length, the singular point is at 1.2 of the frontal plane for a diameter less than 6 mm, and by 0.6 mm more for a 3 mm diameter. This diameter is already too small to allow economic implementation over a length of 4 mm. It is also quite disadvantageous to remove the mass of the magnet from the OZ axis on which one can produce maximum modulation; but this is inevitably produced with tubular magnets of the prior state of the art. In other words one must reduce, insofar as possible, the volume of the hole with respect to the volume of the magnet, which is far from the case in existing devices. In magnet sensors and Hall-effect gaussmeters there is great interest in using Hall-effect gaussmeters which are as thin as possible in order to reduce the air gap between the magnet and the toothed surface, for example the Ashl HZ106C which has a total thickness of 0.6 mm and whose sensitive surface is 0.2 mm from the lower side. The point of the null field being arranged too far from the side of the magnet, the Hall-effect gaussmeter must be moved further from the magnet which requires an increase of the air gap and causes a reduced sensitivity of the sensor. The sensor according to this state of the art is therefore not compatible with practically feasible miniaturization. In addition, control of the exact position of the Hall-effect gaussmeter with respect to the cancellation zone of the field component along OZ is extremely delicate when the field gradient is increased, and this control is hardly compatible with the proposed devices.
The patent U.S. Pat. No. 5,210,489 of the prior art describes a device which includes a magnet that has a threshold whose purpose is to homogenize the field and to make the flux lines parallel. Homogenization of the homogeneous field zone is a goal which is fundamentally different from that of the present invention, in which we are looking for a null field in the zone in which the Hall-effect gaussmeter is placed.